OTA or Over the air is update Huawei P9 Lite Smart DIG-L21 / Diego-L21 Rom from the manufacturer developer fix problems or update from previous firmware, you can install via FOTA updater.apk or recovery like Stock recovery, CWM, TWRP or Carliv

Full OTA is full OTA binary image that allow you to manually update Huawei P9 Lite Smart DIG-L21 / Diego-L21 . This like flashing because usually this firmware is used for repair firmware or unbricking Huawei P9 Lite Smart DIG-L21 / Diego-L21 by some users.